Glacier Bay National Park & PreserveСтраница 35 из 97
Many Glacier Bay Halibut Stay Close to Home, Tagging Studies Show
When researchers released the first acoustic-tagged halibut in Glacier Bay National Park and Preserve in 1991, they expected it to swim away within hours, and they were ready to follow it with a research vessel around the clock for several days. It stayed near the release site for days that turned into weeks and months. Julie K. Nielsen and Andrew C. Seitz of the University of Alaska Fairbanks describe what that fish, and a second tagging study that began in 2013, showed about how halibut use a marine protected area.
Why a halibut study in a park
Pacific halibut is one of the largest flatfish, reaching lengths of almost nine feet (2.7 m) and weighing up to 500 pounds (227 kg), and commercial, sport and subsistence fisheries all seek it. In Glacier Bay, where commercial fishing has gone on since the early 1950s, it is one of the most important commercially fished species. In 1990 the Alaska Wildlife Alliance sued the National Park Service over that fishing. The authors note that commercial extraction is not allowed in parks unless a park's enabling legislation authorizes it, which Glacier Bay's did not for commercial fishing, and that motorized vessels were barred from portions of the park designated Wilderness Waters under the Alaska National Interest Lands Conservation Act of 1980. The suit started a discussion of closures. In 1999 Congress closed all commercial fisheries except charter fishing in the interior waters known as Glacier Bay Proper, and the commercial halibut fishery is being phased out by letting existing fishers continue until they retire, which is estimated to happen between 2040 and 2050. In 2009 Glacier Bay was designated a marine protected area (MPA).
Closing an area to fishing can increase the biomass, size and diversity of fish, but knowing whether a closure will help takes information on how the fish move: the benefits can occur in a protected area large enough to hold a fish's typical movements. When closures were first discussed in the early 1990s, very little was known about small-scale halibut movements, and because halibut are strong swimmers capable of long distances, an MPA was not expected to work as a halibut management strategy.
The 1991 study
To fill that gap, park researchers began an acoustic telemetry project in 1991. A tag on the fish sends out a ping, and when a hydrophone on a vessel or a mooring detects it, the fish's position can be estimated. After three years of tracking 43 acoustic-tagged adult halibut, the results showed a strong tendency to stay in very small areas: most limited their movements to a radius of 0.6 miles (1 km). Some left their "halibut homes" for a while but returned, showing they can navigate back to places they had occupied. The authors say the results suggested that once commercial fishing is phased out, the MPA is likely to protect adult halibut from commercial harvest in summer.
The 2013 study
Adult halibut are known to migrate long distances in winter to spawn, and the commercial fishery runs approximately nine months of the year, so the researchers needed to know where fish go year-round. In 2013 they attached 25 pop-up satellite archival tags (PSATs) to adult halibut 3.4 to 5.6 feet (1.0 to 1.7 m) long, caught on longlines inside the MPA in summer. A PSAT is an external tag, attached with a dart and tether, that records depth, temperature and light until it is programmed to release, then floats to the surface and sends its data to satellites, which lets researchers infer where the fish was each day. Ten were set to release in winter and 15 a year after tagging. Fifteen of the fish also received a surgically implanted acoustic transmitter and were tracked every other week for two months in summer 2013 and again in June and July 2014, with a detection range of 0.3 miles (500 m) and locations generally precise to 492 feet (150 m).
The authors call the summer behavior remarkably similar in the two studies:
| 1991 study | 2013 study | |
|---|---|---|
| Fish in the movement-radius analysis | 27 | 12 |
| Average movement radius | 1,315 feet (401 m) to 1,693 feet (516 m) | 1,591 feet (485 m) to 2,654 feet (809 m) |
| Individual movement radii | 341 feet (104 m) to 4,902 feet (1,494 m) | 282 feet (86 m) to 5,594 feet (1,705 m) |
Winter, and the next summer
Of 21 fish with PSATs that provided data, 15 stayed inside the MPA through the winter. Six left in December, and four of them were back by mid-March. The migration largely coincided with the winter closure of the commercial fishery, so tagged fish were open to commercial fishing outside the MPA for only a very small share of their migration time. Year-to-year movements proved as small as those within a summer. One migrant returned to within 0.6 miles (1 km) of its release location after a spawning trip outside the MPA, likely somewhere in the Gulf of Alaska, and for 14 fish the median distance between the release location and the following summer's location was 0.8 miles (1.3 km), ranging from 0.2 miles (0.3 km) to 16.5 miles (26.5 km).
What it could mean for management
Both studies indicate that Glacier Bay halibut set up small homes that they may leave but can return to, a behavior like that of lingcod and certain rockfish, though at slightly larger scales. The authors say more work is needed on how movement patterns relate to region, habitat and halibut size, and on how common the behavior is. If small-scale site fidelity proves common across the species' range, halibut populations may have more spatial structure than currently thought, which matters for stock assessment and for the potential for local depletion.
The studies also found that most tagged fish did not join spawning migrations. The authors say research should determine whether those fish skip spawning or spawn inside the MPA, because if many adults do not spawn every year, current spawning biomass is overestimated. If halibut elsewhere behave the same way, MPAs could help conserve spawning biomass, and the Glacier Bay MPA, together with the winter closure of the fishery, could provide year-round refuge from commercial harvest for both resident and migrant fish. More than one management option may be valuable, the authors add, because halibut biomass dropped by 50% from 1996 to 2011 but has since stabilized.
Sources
- Julie K. Nielsen and Andrew C. Seitz (University of Alaska Fairbanks), Marine Protected Area Research in Glacier Bay Leads to Insights into Halibut Behavior, Alaska Park Science 19(1), 2020, pp. 18-25, National Park Service; rewritten in hubnx's own words.